This invention relates to devices for reducing in-band interference in r.f. received signals.
Two of the main sources of interference in such signals (e.g. high speed serial data (2.4 k/bits per second) over an HF communications channel), are interference from other users and multi-path propagation.
Multi-path propagation causes intersymbol interference on digital communication links. Intersymbol interference occurs because each bit of transmitted information arrives at the receiver several times spaced over a wide (of the order of 8 milliseconds) differential time interval. At 2.4 k/bits per second, each bit lasts 0.417 milliseconds, so the transmitted information becomes confused and corrupted at the receiver due to the multiple propagation paths.
There are two techniques known for combating this type of problem. The first is channel equilisation, and this works by passing the received signal through a filter which has a response equivalent to the inverse of the channel through which the signal was received. A disadvantage of this type of system is that it is not always possible to produce such an inverse of the channel.
The other known technique for combating the problem is a channel estimator (e.g. WO-A-88/09591) which works by identifying the response of the channel, and the detection of the data takes place by comparing the received (distorted) data with a locally generated data stream that has been passed through a model of the channel.